1. Field of the Invention
The present invention relates to a pattern recognition apparatus, a pattern recognition method, and a computer-readable storage medium storing a program for causing a computer to implementing the method.
2. Description of the Related Art
There are pattern recognition techniques that detect and identify a predetermined pattern from an input pattern. With improvements in computer performance, pattern recognition techniques have also enhanced, and recently, systems that detect a face in an image in real time have emerged. Also, there are known face recognition techniques that detect a face region in an image and check who is a person in the face region.
The accuracy of the face recognition techniques depends a great deal on registration conditions of registration data. Examples of the registration conditions include lighting environment, variations in head pose, changes in facial expression, effects of occlusion, characteristics of an image pickup device, the number of registered images for each registered person, and the number of registered persons.
Several techniques have been proposed to deal with variations in these registration conditions. A description will now be given of the reason why variations in the number of registered images affect, which are exemplary variations, face recognition accuracy, and a technique that has been proposed.
First, a description will be given of the effects which the number of registered images has on face recognition accuracy. When a person other than registered persons is to be checked, it is highly probable that an image similar to the person exists in registered images for a registered person for which there are a number of registered images. In this case, the similarity between the registered person for which there are a large number of registered images and the other person is high on average. Such variations in similarity affect a false acceptance rate indicative of the probability at which another person is falsely identified as a registered person. A technique to reduce this effect is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-149074.
According to Japanese Laid-Open Patent Publication (Kokai) No. 2005-149074, a checking algorithm or a feature amount is dynamically changed according to the number of registered images so as to enhance robustness against variations in the number of registered images. To dynamically change the checking algorithm, distances between a distribution obtained from a plurality of personal images and an input image are recognized based on similarity. On the other hand, when the number of registered images is small, recognition is performed based on a similarity between one personal image and an input image. It is described in Japanese Laid-Open Patent Publication (Kokai) No. 2005-149074 that the greater the number of registered images, the more accurate the expression of a personal distribution, and thus, in this case, by performing a statistic checking method, recognition accuracy can be enhanced. Moreover, it is described that regarding the way in which a feature amount is changed, when the number of registered images is small, luminance data on each image is used as a feature amount. On the other hand, it is described that when the number of registered images is large, a mean vector after projection, which has been projected to a predetermined eigenspace, is used as a feature amount.
According to Japanese Laid-Open Patent Publication (Kokai) No. 2005-149074, a checking algorithm or a feature amount is changed according to the number of registered images for a registered person, but this is not preferable in terms of resources because a plurality of checking means and feature amount extracting means have to be prepared for mounting.